Steel cables for tires as a rule consist of pearlitic (or ferrite-pearlite) carbon steel wires, hereinafter “carbon steel”, with a carbon content generally between 0.2% and 1.2%, the diameter of these wires being most often between about 0.10 and 0.40 mm. These wires are required to have very high tensile strength, generally higher than 2000 MPa and preferably higher than 2500 MPa, obtained thanks to the structural hardening that takes place during the cold drawing of the wires. The wires are then assembled in the form of cables or strands, and this also requires the steels used to have sufficient torsional ductility to withstand the cabling operations.
In a known way, tires for industrial vehicles such as heavy vehicles usually comprise a carcass reinforcement anchored in two beads and surmounted radially by a crown reinforcement comprising one or more working crown plies and one or more crown protection plies over the working crown ply or plies, the said crown reinforcement itself being surmounted by a tire tread joined to the beads by two sidewalls.
The protection plies, which can be deformed thanks to a degree of elasticity, are mainly intended to impede the penetration of foreign bodies radially into the tire during rolling.
To reinforce the protection plies of tires for industrial vehicles such as heavy vehicles, nowadays it is customary to use strand cables (or strand “cords” in English), which are ssembled by the known technique of stranding and consist, by definition, of a plurality of metallic strands twisted together in helix, each strand comprising steel wires also twisted together in helix. Note that most of the wires used in these cables for crown protection plies have a diameter typically above 0.20 mm, for example close to 0.30 mm, this diameter being in particular larger than that of the wires used in the carcass reinforcement of heavy-vehicle tires.
As strand cables currently used to reinforce the crown protection plies of heavy-vehicle tires, one may mention for example a cable with the formula (4×2), consisting of 4 strands twisted together in helix, each consisting of 2 metallic wires twisted together in helix.
The cables for crown protection plies are designed on the one hand to confer optimum flexibility on the ply containing them, so that the crown protection ply can best conform to the shape of an obstacle it encounters during rolling, and on the other hand to allow the ply to resist the penetration of foreign bodies radially into it.
Note also that these strand cables must be as thoroughly impregnated as possible by the rubber, so that the latter penetrates into all the spaces between the wires constituting the cables. In effect, if this penetration is insufficient, empty channels form along the cables and corrosive agents such as water, which can penetrate into tires for example due to cuts or other aggressions of the tire's crown reinforcement, make their way along those channels through the said reinforcement. The presence of this moisture plays an important part in causing corrosion and accelerating the processes of fatigue (“fatigue-corrosion” phenomena) compared with use in a dry atmosphere.
On the other hand, each bead of a tire for a heavy industrial vehicle is generally provided with a stiffener designed to reinforce it, each stiffener comprising at least one ply of cables extending axially on the outside of the upturn of the carcass ply. These cables make an angle generally between 15° and 30° relative to the circumferential direction of the tire. In some cases each stiffener can extend along the upturn of the carcass ply both axially outside and radially inside the upturn, essentially forming an L when viewed in the meridian cross-section of the tire.
These stiffeners have an overall damping function which tends in particular to minimise the phenomenon of “deradialisation” of the carcass reinforcement of the tire each time it is flattened against the ground over which it is rolling, by taking up the compression forces connected with the flattening and minimising the wear against the rim.
To reinforce the plies, constituting these stiffeners, the cables most often used are layered steel cables consisting of a central core and one or more concentric layers of wires arranged around the core.
The most widely used layered cables in these stiffeners for heavy-vehicle tires are ones having the formula (L+M) or (L+M+N). As is known, these cables are formed of a core of L wire(s) surrounded by at least one layer of M wires itself sometimes surrounded by an outer layer of N wires, L generally varying from 1 to 4, M from 3 to 12, N from 8 to 20, depending on the case, and the whole if necessary being surrounded by a wrapping wire wound around the last layer.
Like the cables for crown protection plies, it is also important that these layered cables for stiffeners are as fully impregnated as possible by the rubber, so that it will penetrate between the wires constituting the cables to prevent corrosive agents from making their way into empty channels along the cables.
For example, layered cables of structure (3+9+15) have been described, which consist of an inside layer of 3 wires surrounded by an intermediate layer of 9 wires and a saturated outer layer of 15 wires, for example provided with a wrapping wire, as described in the documents EP-A-176 139 (or U.S. Pat. No. 4,651,513), EP-A-497 612(or U.S. Pat. No. 5,285,836), EP-A-669 421 (or U.S. Pat. No. 5,595,057), EP-A-709 236 (or U.S. Pat. No. 5,836,145),EP-A-744 490 (or U.S. Pat. No. 5,806,296) or EP-A-779 390 (or U.S. Pat. No. 5,802,829).
A major disadvantage of these cables with formula (3+9+15) is that they cannot be penetrated throughout by the rubber, in particular because of the saturated structure of the outer layer and the presence of a channel or capillary at the centre of the three core wires, which remains empty after impregnation by the rubber and is therefore available for the propagation of corrosive agents such as water.